This invention relates to the measurement of temperature of frozen tissue, and more particularly the invention relates to non-invasively measuring the temperature using Magnetic Resonance Imaging (MRI) signal measurements.
The measurement of temperature in frozen tissue is important in many fields including food science, cryobiology, and cryosurgery for example. In cryosurgery diseased tissue is treated by lowering the temperature of the diseased tissue. The monitoring of the temperature of the tissue is important so that normal tissue is not damaged while the diseased tissue is killed. The safety and efficacy of minimally invasive cryosurgery increased substantially with the use of ultrasound to monitor ice ball formation. However, recent studies of prostate cryosurgery indicate that to achieve high success in eradicating tumors, lesions must not only be frozen, but should be cooled to a lethal temperature of approximately -40.degree. C. Because ultrasound cannot interrogate the interior of the ice ball, cryosurgeons have had to rely on invasive, percutaneously inserted thermocouples to monitor temperatures at a few selected points within the ice ball to monitor the freezing process.
Existing methods for monitoring of temperatures in frozen tissue includes probes, such as thermocouples, fiber optic sensors, or thermistors that are physically inserted or in contact with the tissue. During cryosurgery, this implies physically penetrating the target tissue with multiple thermal probes, resulting in a more invasive procedure. Furthermore, because MR imaging can provide a two- or three-dimensional map of temperature dependent signal changes, the distribution of temperatures in a frozen sample can be more accurately assessed.
Theoretical calculation of temperatures, based on heat transfer models have been used. Recently, it has been proposed that MR could be used to predict thermal gradients within tissues based on heat transfer models, and knowledge about probe temperature and tissue freezing properties. These methods are limited, however, because they merely infer the thermal gradient within the ice ball, rather than measure it directly. It is unclear how well they will perform in clinical cryosurgery where multiple probes are used, and where the freezing duty-cycles of various probes are continuously adjusted to control the shape of ice ball formation, and hence heat transfer in the ice ball is not at a steady state.
The use of MRI has been proposed for assisting in cryosurgery in the following references:
Pease, G. R. et al., MR Image-Guided Control of Cryosurgery. JMRI 5, 753-760 (1995).
Matsumoto, R. et al., Monitoring of Laser and Freezing-induced Ablation in the Liver with T1-weighted MR Imaging. JMRI 2, 555-562 (1992).
Gilbert, J. C., MR Analysis for Assessing the Temperature Distribution in a Cryosurgical Frozen Region. (Abstract) Proceedings of International Society for Magnetic Resonance in Medicine Fourth Scientific Meeting and Exhibition 3, 1746 (1996).
Hong, J S, MR Imaging Assisted Temperature Calculations During Cryosurgery. MRI 12, 1021-1031 (1994).
U.S. Pat. No. 5,433,717 discloses magnetic resonance imaging assisted cryosurgery employing T1 measurements to determine temperature distribution in unfrozen tissue regions.
The present invention is directed to measuring temperatures within the ice ball of frozen tissue during cryosurgery and provide non-invasive and more comprehensive monitoring of cryosurgery, in one illustrative application.